Electronic amplifiers and mixers are used in countless communication applications, of which transmitters, radio and television receivers, mobile telephones, and electronic test instruments such as spectrum analyzers are but a few examples. Signals propagating through such an amplifier or a mixer are subject to various kinds of distortion of which third-order intermodulation (IM) distortion can be particularly difficult to eliminate.
One way to reduce 3rd-order IM distortion is to generate a compensating signal having 3rd-order IM distortion that opposes the distortion introduced by the amplifier or mixer. Combining such a compensating signal with a signal passing through an amplifier or a mixer can reduce the distortion to an acceptably low level. Generators that provide such compensating signals have found an application in transmitters. An example is described in the paper “Linearized transponder technology for satellite communications”, part 1 by D. Cahana, J. R. Potukuchi, R. G. Marshalek, and D. K. Paul, and part 2 by Y. S. Lee, I. Brelian, and A. Atia, COMSAT Technical Review, Volume 15 Number 2A, Fall 1985, pages 277-333. This paper describes a 3rd-order IM generator using unbiased diodes to correct for distortion in a traveling-wave-tube power amplifier.
Another technique uses a weakly distorting device such as a biased diode or transistor to generate a 3rd-order output.
This and other examples of such devices may be found in the text High-Linearity RF Amplifier Design by Peter B. Kenington, Chapter 6 (pages 351-424), ISBN 1-58053-143-1, published 2000 by Artech House, Inc., Norwood, Mass.
Techniques such as those described above and in Kenington have been useful primarily in systems having limited ranges of amplitude or frequency. Accordingly, there remains a need for way to compensate for 3rd-order IM distortion in applications such as superheterodyne receivers that must operate over a range of input frequencies and signal amplitudes.